Power plant water issues: Effectively cleaning cooling tower fill

While once-through cooling was a common feature at many power plants in the last century, environmental regulations regarding intake and discharge issues have basically forced a transition to cooling towers, or in some cases air-cooled condensers, for new projects.  Essential to steady heat transfer in cooling towers, and also their physical stability, is proper chemistry control.  But even with diligent chemical feed and monitoring, cooling towers, and especially the tower fill, can accumulate scale and microbiological deposits that inhibit heat exchange, and, in worst case scenarios, may induce partial collapse of the tower.  This article examines methods to clean tower fill before fouling causes irreversible damage.

Background

Cooling tower performance is highly dependent on the efficiency of contact between the hot return water from heat exchangers and the cool air being pulled or blown through the tower.  Heat transfer is enhanced by the use of cooling tower fill, which, over the decades, has evolved into sophisticated designs to maximize air-water contact.  An illustration of high efficiency PVC film fill is shown below.

 

 

Figure 1. High efficiency cross-fluted fill (Photo courtesy of Brentwood Industries)

 

The transition from early splash fill designs to modern high efficiency types reduced cooling tower capital and operating costs.  However, the generally tortuous path that provides good contact between air and water also makes these fills highly prone to fouling.  Advanced fouling results in a ~10x weight gain, leading to fill collapse into the sump and expensive fill replacement. 

Proper Chemical Treatment

While this article focuses on methods to clean tower fill that has begun to accumulate deposits, it is paramount to understand that proper chemical treatment during normal operation is essential to prevent severe or sudden scaling and fouling problems.  With regard to scale (and corrosion) control in cooling systems, the four-decade long methodology of phosphate/phosphonate treatment is giving way to polymer-based, non-phosphorus chemistry for two primary reasons.  One is that phosphorus discharges to the environment are being increasingly regulated and restricted due to problems with toxic algae blooms that have afflicted numerous bodies of water.  Secondly, the new polymer programs are proving to be more effective for scale and corrosion control in cooling systems.  (Post, R., Kalakodimi, R., and B. Buecker, “An Evolution in Cooling Water Treatment”; PowerPlant Chemistry Journal).

The most serious issues in cooling systems are usually related to microbiological fouling.  Thus, virtually all systems have as primary treatment some form of oxidizing biocide, most commonly bleach but also possibly gaseous chlorine, bleach/sodium bromide, chlorine dioxide, monochloramine, and monobromamine.  A problem at many facilities, and this is particularly true in the power industry, is that the regulations developed for and by the United States Environmental Protection Agency (USEPA) allow no more than 0.2 ppm free available chlorine average residual for 2 hours per day as “Best Available Technology.”  For plants so constrained, treatment is only allowed for less than 9 percent of any day, thus giving microbes a chance to settle and begin forming protective slime layers.

 

Figure 2

 

Options for dealing with fouled fill

Many facilities have suffered from fouled cellular plastic fill.  Replacement of the fill in kind is a potential solution, but potentially sets up repeat situations.  Another option is a switch to low-fouling fill designs that generally feature a more vertical flow pattern, less surface texturing, and sometimes wider spacing between the plates, all at the expense of some cooling efficiency.  Since fill replacement can be expensive in terms of both materials and outage time, others have chosen to clean the fill chemically, or sometimes, mechanically.  The choice of replacement vs. cleaning, as well as the cleaning methodology, requires careful consideration.  The decision depends on the extent of the fouling, the physical and chemical nature of the foulant, the type of fill, and environmental considerations in dealing with cooling tower blowdown.  For example, in heavily fouled film packs, some passages may be completely blocked, preventing the cleaning solution from flowing through, and perhaps acting as a filter for solids removed in other parts of the pack.  The total mass of deposits, if released at once into the recirculating water flow will result in very high suspended solids, and blowdown may have to be diverted or treated prior to discharge.  The type of foulant also varies considerably depending on the nature of the circulating water and the treatment chemistry employed.  Over time, the fouling matrix behaves as a filter media, trapping additional suspended solids in the crevices of the fill pack and impeding air and water flow.  At this point, the efficiency criteria that constituted the driving force for selecting the fill has become null. 

Figure 3.  Fouled film fill that is no longer effective.

The loss in cooling tower capability as a function of weight gain for a fill of offset flute design is trended in Figure 4.

 

Figure 4.  Tower capability loss vs. fill weight gain for a standard offset flute cellular plastic fill pack. (Monjoie, Michel, Noble, Russell, and Mirsky, Gary R., Research of Fouling Film Fill.  Cooling Technology Institute, TP93-06, New Orleans, LA, 1993.)

 

Over time, the high efficiency fill becomes increasingly less efficient, may gain as much as 10x its initial weight, begins to extrude around the supporting beams, and ultimately collapses into the sump.  At the point where performance loss becomes obvious to operators or the fill begins to deform, it is too late to consider cleaning as an option; fill replacement is required.  However, if the fouling is detected in its early and moderate stages, several cleaning options are available, depending on the nature of the foulant.

Cleaning Options For Cellular Plastic Fill

The most appropriate method for cleaning tower fill depends on several factors, including safety concerns, system metallurgy, in-service vs. out-of-service cleaning, potential impact on plant operations, disposal options for the cleaning solution, impact on the environment, and the chemical and physical nature of the foulant.

Mineral Scales

Hard mineral deposits most commonly consist of silica/silicates or calcium carbonate (calcite).  Silica solubility is lowest at low temperature, and deposits often occur near the bottom of the counterflow fill pack where the temperature is lowest, the water is most concentrated, and uneven water/air distribution can lead to dry spots or locally concentrated areas.  Calcite deposits often occur throughout the fill pack, but are generally heaviest toward the bottom.  Higher temperature near the top of the fill pack has the lowest calcite solubility and promotes faster deposition kinetics.  However, as the water passes through the fill, the minerals are concentrated slightly by evaporation, and the pH will rise slightly as excess CO2 is stripped.

One technique that can be used effectively on either type of hard scale in its early stages is to apply certain types of surfactants that penetrate the hard deposit and induce it to spall from the slightly flexible plastic substrate. The surfactant is typically applied in addition to the normal scale inhibitor program for an extended period of 60-180 days. This program is never 100% effective, but will often result in removal of 70-80% of the fouling minerals.  Prior to implementing the cleaning process, it is imperative to identify and correct the scaling condition.

For large cooling systems, where the predominant scale deposit is calcite, the fill can be cleaned by reducing the operating pH and/or cycles of concentration until the water is undersaturated with respect to calcite at the fill conditions.  Calcite often serves as the binder for the deposit matrix, so dissolving the calcium carbonate in the deposit matrix can be disproportionately effective.  In principle, any degree of undersaturation will be effective over time.  Sulfuric acid is an obvious choice for many plants that already use it for pH control, but very careful planning involving plant personnel, the chemical supplier, and any outside contractors is required before using such a hazardous chemical. Other plants may prefer to use safer, less corrosive acids such as organic acids or inhibited sulfamic acid. [4]  Some organic acids are more effective than mineral acids at intermediate pH, and are synergistic with sulfuric acid.  At pH 5, application of the appropriate organic acid will accelerate the rate of calcite dissolution by 10-20x as compared to sulfuric acid alone.

For predominantly light calcium carbonate scaling, off-line foam acid cleaning has been used very successfully, at least on smaller towers. Strong acid foam is applied by skilled specialists from the top of the fill pack. The nature of the foam allows the acid to contact the scale as it slowly passes downward through the fill.  The relatively low volume of spent and mostly neutralized foam cleaning solution is either collected in the sump and disposed of, or is allowed to mix with other circulating water from neighboring tower cells that may be in service, depending on plant safety and environmental requirements.

Mineral scales can also be mechanically cleaned with some success in situ or ex situ.  Due to its brittle nature relative to the flexible PVC, the scale can be dislodged with some success by mechanically cleaning the fill pack in-situ from below.

Microbiological/Organic Deposit Matrices

Deposits where microbiological growth or organics serve as the binder for the deposit matrix are characterized by a soft, sometimes putty-like consistency.  Unlike mineral scales, deposits of microbiological origin tend to accumulate primarily in the middle of the fill pack.  Water velocities directly under the spray nozzles are generally high enough to discourage microbiological adhesion.  For this reason, microbiologically initiated fouling sometimes goes undetected because it is not visible on inspections from the top looking down beneath the spray headers.  As the water velocity slows down several inches into the fill, microorganisms begin to colonize the surface, acting as a filter for suspended solids passing through the fill.  Fouling tends to be more intense in the middle of the fill than at the bottom because suspended solids are filtered out prior to reaching the bottom layer, and because the last few inches of fill do not physically support a thick, soft deposit mass.  The inability to clearly view microbiological fouling from either top or bottom, combined with the difficulty of inspecting the middle layers of fill, often allows this type of fouling to progress undetected until it has reached an advanced stage.  Plant personnel have attempted to monitor fill fouling during tower operation using sections of fill suspended from load cells, or by cutting an access window into the end of the tower casing to allow a middle section to be removed periodically for inspection using a man lift, or by suspending a section of fill beneath the main fill pack to allow it to be easily inspected and weighed.  All of these methods can work, but none have proven to be totally satisfactory.

Several effective methods exist to remove biological-silt matrix deposits from cooling tower fill.  Hyperhalogenation is a widely attempted method, but its effectiveness is usually disappointing.  Potential corrosion of system components and the need to dechlorinate prior to discharge are important considerations.

Microbiological matrices often have high water content and will shrink and detach from surfaces when thoroughly dried.  However, effectively drying out cooling tower fill can prove problematic even with the help of fans, even if the tower is located in a low humidity climate.  Chlorine dioxide has also been used as a cleaner for cooling tower biofilms with some success.  However, the most widely practiced and effective cleaning method for deposits with microbiological or organic binders is hydrogen peroxide (H2O2) due to its oxidizing strength and the physical action of the oxygen micro-bubbles produced as the chemical reacts with organic deposits.  The positive environmental profile of hydrogen peroxide involving rapid breakdown to water and oxygen, and its ease of application are additional factors favoring peroxide as a tower fill cleaner.  Typical dosages are in the range of 500-3,000 ppm active H2O2.  As with most cleaning operations, the addition of low levels of surfactants will help loosen deposits.  Polymeric dispersants are generally added to assist in keeping the removed solids in suspension until they can be blown down.

Much of the biomass consists of extracellular and intracellular water and organics that will dissolve with peroxide cleaning.  A substantial portion of the deposit typically contains much mud and silt that will be released into the water.  Figures 5 and 6 illustrate the appearance of a slime-clay matrix on moderately fouled high efficiency cooling tower fill before and after cleaning. 

 

 

In cases where the deposit contains a high percentage of inorganics, the circulating water can be expected to become highly turbid.  The potential for high suspended solids in the cooling tower blowdown should be anticipated when cleaning a severely fouled system and taken into account in the job planning scope.

Summary

  • Every effort should be made to prevent deposition from occurring in the first place.  A fill type should be specified that is compatible with reasonable expectations for the system, considering influent water quality, microbiological control, presence or absence of pre-treatment equipment, and the possibility for external foulants that might enter the tower through airborne contamination or process fluid leaks. 
  • The microbiological and deposit control program should be diligently monitored to ensure that it is within expectations and delivering the required results. 
  • The performance of any pretreatment and sidestream solids removal equipment should be reviewed to ensure such equipment is delivering and maintaining suspended solids within specifications.
  • Plant personnel should be proactive with inspection and monitoring.  There are more options, and less expensive ones, if the fouling is detected at an early stage.
  • Periodic, light, preventative maintenance tower fill cleanings should be considered.  Most high efficiency fills tend to gain weight slowly over time. Annual preventative maintenance cleanings can stabilize or reverse that trend.
  • If cleaning is indicated, safety and environmental considerations must be adequately addressed.  Cleanings require careful planning and coordination between plant personnel and the chemical supplier/consultant.  Personnel safety is critical.  Also, cleanings can release many suspended solids that if not carefully controlled can foul equipment or present disposal problems without proactive planning.
  • If fouling occurs, all remediation options should be considered, but, generally, the least costly and least aggressive methods applicable to the nature and quantity of the deposit should be the starting point.  Identifying and correcting the fouling conditions at an early stage is least expensive, and preferable to aggressive cleaning or ultimate fill replacement if the fouling conditions are allowed to persist.
  • All systems are different, and careful consultation with the cleaning vendor and water treatment experts should be a priority.

 You can find the original article @ https://www.power-eng.com/2019/08/21/power-plant-water-issues-effectively-cleaning-cooling-tower-fill/#gref

HOSPITAL REDUCES WATER USAGE IN COOLING TOWERS WITH AUTOMATION

With the state's initiative to reduce water usage by 20 percent by the year 2020, many plants in California are striving to become more environmentally friendly. One such facility includes a leading California hospital that sought to reduce water treatment costs for its HVAC system. The hospital has three individual cooling tower systems that service three centrifugal chillers, with a combined total of 2,800 tons of capacity.

The water treatment program currently in use at the facility was operating at 2.8 cycles of concentration, resulting in 35.7 percent of the tower water makeup being bled to the sewer by the current treatment provider. Given the water quality in the area, this was the maximum cycles of concentration that could be achieved without employing the use of acid or water softening.

The savings that the hospital sought were realized by reconsidering various ways to optimize the water treatment program. Working closely with the Los Angeles Department of Water and Power (LADWP), it was revealed that by introducing a water conservation program to reduce water use through increased cycles of concentration, the facility would actually save more money than it would spend to alter the program, making the proposed project sustainable.

Through testing and lab analysis, the team was able to conclude that six cycles of concentration could be attained, resulting in only 16.7 percent of the tower makeup water being bled into the sewer treatment system. This could be achieved through the introduction of a safe acid feed system that would minimize scale, corrosion and microbiological fouling to enable the increase in cycles of concentration while also protecting facility staff from coming into contact with the chemicals.

The evaporation of the cooling tower remained the same, but U.S. Water was able to reduce blowdown, cutting water usage by an estimated 3.6 million gallons per year and decreasing water and sewage costs. The plant was able to save over $76,000 (see Fig. 1).

Anytime chemistry in the cooling tower is stressed by adding more cycles, tight control of the chemistry is required to prevent scale formation. This led to the introduction of U.S. Water's advanced automation controls. The advanced automation program included wireless monitoring and alarm notifications to manage the overall program performance, and the equipment monitored conductivity, pH, scale inhibitor levels, tower makeup usage, and tower bleed usage.

At any given time, designated hospital personnel and U.S. Water representatives, using various levels of password-protected security outlined by the facility, can securely access the data for review and online adjustment. If designated parameters fell above or below the specified range, a U.S. Water representative was alerted for quick response (see Fig. 3).

Second to irrigation, cooling towers offer the largest potential for water savings in California. As an added incentive, the state of California has put programs in place to rebate facilities for the cost of automating their systems. LADWP and the Metropolitan Water District (MWD), for example, offer three programs that finance automation for cooling towers due to their ability to increase cycles of concentration, which reduces water use.

This financing allowed U.S. Water to implement the $34,000 advanced automation program to monitor and control the water treatment program for this hospital at no cost to the hospital.

Results to date for the facility include significant reduction in water usage, lower water and sewage bills and more efficient monitoring due to the installed automation software to protect the equipment assets.

You will find the article at: http://www.waterworld.com/articles/iww/print/volume-14/issue-5/columns/case-study/hospital-reduces-water-usage-in-cooling-towers-with-automation.html

Boiler Carryover – Cause, Effect and Prevention

Mechanisms

carryover or primingCarryover also known as priming is any solid, liquid or vaporous contaminant that leaves a boiler with the steam. In low/medium pressure boilers (<100 bar) entrained boiler water is the most common cause of steam contamination.

Both mechanical factors such as boiler design, high water levels, load characteristics and chemical factors such as high solids concentration, excessive alkalinity, presence of contaminants contribute to the creation of carryover.

Two of the most common mechanical causes of carryover are operation in excess of design load and sudden increases in load.

Foaming is one of the mechanisms of chemical carryover. Foaming tendencies are increased with increases in alkalinity and solids content. Stable foam bubbles contain boiler solids and are carried forward with the steam giving rise to carryover.

Oil and other organic contaminants can react with boiler water alkalinity to give crude surface active materials which cause foaming and carryover.

Effects

Boiler water solids carried over with steam will form deposits in non-return and other control valves. Process streams can be contaminated by carryover affecting product quality.

Deposition in superheaters can lead to failure due to overheating and corrosion.

Steam turbines are potentially prone to damage by carryover as deposits on turbine blades creates imbalance reducing efficiency and capacity. Solid particles in steam can lead to erosion and corrosion in both turbines and other equipment.

 

Prevention of Carryover

The prime means of preventing carryover is to have good mechanical steam separation devices. For low/medium pressure fire tube boilers where steam purity is not stringent, gravity separation is normally satisfactory. (At least 14 bar and saturation conditions the density of water is 115 times greater than that of steam). As steam pressure rises the difference in density reduces (at 69 bar water is only 20 times more dense than steam) making gravity separation less effective. Steam separators are then used to improve purity and are usually installed in the steam drum of water tube boilers.

Primary separators utilise the difference in density as the means of separation bypassing steam through a series of baffles which reduces turbulence or centrifugal (cyclone) separators.

Secondary separators, where steam is directed in a frequently reversing pattern through a large contact surface. A mist of boiler water collects on the surface and is drained from the unit.

Control of boiler water chemistry is essential to minimise carryover and allow mechanical separation to work effectively. The parameters that must be controlled are:

  • Total dissolved solids
  • Alkalinity
  • Silica
  • Organic contamination.

These should be maintained within the boiler manufacturer guidelines or those of BS 2486.

Whenever carryover is being caused by excessive boiler water concentrations an increase in boiler blowdown rate is normally the simplest and most expedient solution. If carryover is still occurring and increasing blowdown is uneconomic then the addition of antifoam agents can economically reduce carryover. Use of an antifoam may allow the boiler to operate at higher water concentrations, Feedwater offer a product called Defoamer C which is suitable for this job, for more information visit the product page for product usage guidance.

Read more at https://feedwater.co.uk/boiler-carryover-cause-effect-prevention/

 

Fundamentals of corrosion control in water systems

Liquid analysis systems and sensors are cost effective tools against corrosion.

Water plus metal equals corrosion. This reality attacks the bottom line of every steam driven power generation plant in the world.

In a steam power plant, high purity water is heated and boiled to make steam, which energizes and powers a turbine to produce electricity.

Water and steam are in constant contact with metal surfaces threatening the integrity of plant equipment like condensers, heaters, pumps, piping, boilers, and turbines.

Fortunately, water purification and chemical treatment greatly reduce and control the corrosion in the plant. Ensuring good cycle chemistry to prevent corrosion, however, requires accurate and continuous analytical measurements in the demineralization train, cooling water, condensate, and boiler feed-water and steam systems.

While the guidelines given below address the needs of a steam driven power generation facility, they can also be useful in other manufacturing facilities where water plays an important role.

Corrosion occurs when metal ions transfer from a base metal to water and combine with oxygen to become hydroxides and solid metal hydroxides. Resultant particles often travel to other parts of the system and are deposited.

Rust reaction
Rust reaction

Deposit is a poor conductor

Once a deposit forms, it attracts more suspended solids and the deposit grows. Deposits frequently accumulate on heat exchange surfaces, boiler tubes, and heaters.

The deposit is a poorer conductor of heat than metal and therefore interferes with heat transfer across the tube. This lowers the overall cycle efficiency and can cause local tube overheating failures. Deposits can also significantly lower the efficiency of the turbines and, in turn, become corrosion sites when dissolved solids trapped in the deposit concentrate as the liquid boils away. Eventually, the concentration reaches highly corrosive levels and severe under-deposit corrosion occurs.

A tough oxide film that protects the base metal is the best way to defend iron and copper from corrosion. For iron and carbon steel, the protective film is magnetite.

For copper and copper alloys, the protective film is cuprous oxide. This film works only in the presence of properly controlled water chemistry.

Proper water chemistry also ensures that the film won't wear away and, if a break occurs, the film quickly repairs itself.

Controlling water chemistry requires maintaining high purity water, controlling pH, monitoring for trace quantities of dissolved oxygen, and, if necessary, controlling the feed of a scavenging agent like hydrazine.

Demineralization train

The first line of defense against corrosion in a steam power plant is the use of high purity water. Producing that water is the function of the demineralization train, which converts raw water containing between 100 and 1,500 ppm dissolved solids into water that contains no more than 10 to 20 ppb dissolved solids. Treatment steps may include filtration, softening, chlorine removal, reverse osmosis, degasification, and ion exchange.

Efficient reverse osmosis (RO), in which water forces through a semi-permeable membrane, can remove approximately 98% of the dissolved salts and silica in raw water and nearly all large organic molecules. Contacting conductivity sensors placed in the feed water and the permeate of the RO let plant operators monitor the water quality and overall efficiency of the RO system.

Conductivity measurements in RO permeate and high purity water are not simple, however. Calibration of sensors is complex and must take place by comparing the sensor against a National Institute of Standards and Technology (NIST) traceable calibrated cell of a known cell constant or by calibrating the sensor in a certified solution. However, upon exposure to the atmosphere, high purity conductivity standards and water foul through the absorption of carbon dioxide from the surrounding air and any residue in the sample container. To prevent contamination, it may be desirable to use sensors pre-calibrated to NIST standards. Conductivity validation instruments are available that connect to the process via tubing, eliminating the effects of the atmosphere on the measurement.

Typically, feed-water to an RO system will undergo treatment and will already contain chemicals to ensure optimum operation. These chemicals, however, require careful monitoring, or they may attack the RO membranes. This is particularly true if the feed-water is outside the desired acidic range. Plant operators require general-purpose pH sensors to maintain mild acidity in the feed-water. Chlorine may be in the feed water in some plants as a biocide or need removal in others by means of a carbon bed because it attacks the RO membranes. However, carbon beds reach saturation over time, therefore, chlorine monitors detect breakthrough of chlorine.

Reverse osmosis alone can rarely produce water of sufficient purity for make-up. The RO permeate is usually polished using an Ion Exchanger (IX). These systems consist of tanks containing resin beads selectively treated to adsorb either cations or anions. A cation bed exchanges positively charged ions (such as calcium, magnesium, and sodium) for hydrogen, and the anion bed exchanges negatively charged ions (such as chloride, sulfate, and bicarbonate) for hydroxyl. The displaced hydrogen and hydroxyl combine to form pure water. After a certain amount of use, these systems become exhausted and must be regenerated using sulfuric or hydrochloric acid for cation resin and sodium hydroxide for anion. The monitoring of the concentration of both of these substances must happen continuously with conductivity sensors measuring the regenerant as it enters the tank. During rinse, toroidal conductivity measurements made on the bed effluent determine how well rinsed the regenerants are.

Ammonia, Conductivity, and pH

Variations in cooling tower design

In the condenser, recirculating cooling water converts turbine exhaust steam into condensate. Cooling water usually contains high levels of dissolved solids, and leakage of cooling water into the steam cycle is a major source of contamination.

Leaks introduce ions that raise the conductivity and increase the corrosiveness of the feed-water, boiler-water, and steam. To give early indication of leakage and to monitor the overall condenser performance, the cation conductivity of the condensate pump discharge registers on a flow-through conductivity sensor.

In addition, monitoring condensate and feed-water purity requires measuring cation conductivity. After the condensate passes through the cation column, the conductance of the contaminating salt increases as it converts to a significantly more conductive acid.

There is an increased emphasis in the industry on the re-use of cooling water using cooling towers. The cooling effect comes by the evaporation of a small fraction of water and heat exchange with the air passing through the cooling tower. As the water evaporates, however, the dissolved solids concentrate, ultimately causing scale and corrosion in the heat exchange equipment. While there are many variations in cooling tower design, a common feature is the control of water quality with the use of continuous pH and conductivity measurements to maintain a given set of conditions. A contacting conductivity sensor measures the relative concentration of the impurities in the water. The analyzer for that sensor initiates the opening of a blowdown valve when the conductivity becomes too high. Higher purity make-up water is then introduced which lowers the conductivity.

Since most impurities in cooling water are alkaline, a small quantity of sulfuric acid adds in to the circulating water to lower the pH and thus prevent the formation of scale. Measuring this sulfuric acid concentration and keeping the pH below seven, where scaling is less likely to occur (as indicated by the Langelier Index), is best accomplished by a general-purpose pH sensor. Cooling water that contains a high level of suspended solids, however, requires the use of more specialized pH sensors more resistant to fouling.

Liquid analysis in steam power generation

Condensate feed-water

The cooling tower turns steam into water after leaving the turbine. Make-up water from the demineralization train adds to this water to become feed-water, which pumps through a series of heaters to the boiler. Controlling corrosion in the condensate and feed-water system is usually accomplished in one of two ways-all volatile treatment (AVT) and oxygenated treatment (OT). AVT uses ammonia to control pH and hydrazine to provide a reducing environment for protection of copper alloys. AVT requires measurement of ammonia, dissolved oxygen, and hydrazine. Ammonia measurement can happen either directly or indirectly from pH and conductivity. The indirect method is useful because ammonia reacts in water to produce hydroxide ion. Both conductivity, which is a measurement of ions in solutions, and pH, which is an indirect measurement of hydroxide ion, can combine to yield the ammonia concentration.

OT uses ammonia to control pH and trace oxygen to provide a slightly oxidizing environment that promotes formation of a tough modified oxide film. Water quality for OT is more stringent than for AVT, requiring cation conductivity of less than 0.15 micro Siemens/centimeter. It is necessary to measure dissolved oxygen, pH, and cation conductivity in feed-water systems using the OT method. pH measurement can be difficult in low conductivity water and requires the use of flowing reference technology. A pH measurement requires electrical continuity between the reference and glass electrodes and a path to the solution ground. High purity water does not provide enough conductivity to reliably complete these paths and causes junction potential that registers as erratic drift and offset in the pH measurement. A flowing reference eliminates this effect by stabilizing the junction potential. This measurement takes place in a bypass line in order to preserve the quality of the feed-water and preferably in a stainless steel measurement chamber to dissipate the electrostatic current generated by the high purity water. Since high purity pH is flow sensitive, flow rates should be very low and constant.

Boiler water steam treatment

The boiler is the final collection point for all the corrosive and scale-producing contaminants generated upstream. Solid corrosion lands on the boiler tube surfaces and grows by collecting more suspended matter. Eventually, overheating and tube failure occur. Maintenance of a protective oxide film is the optimum way to limit water corrosion, and this more readily happens when maintaining a low concentration of dissolved solids in a slightly alkaline pH environment. To accomplish this, continuous measurement of both pH and conductivity needs to happen. Conductivity measures the concentration of dissolved solids and a long-life conductivity sensor is required. To maintain the alkaline environment required, power plants commonly buffer the boiler water with sodium hydroxide and sodium phosphate salts. Overfeeding or underfeeding of these chemicals can be damaging, however, and therefore accurate pH and phosphate measurements are critical.

Boiler water also undergoes treatment in order to produce high purity steam. Impurities enter this boiler water from the boiler drum and from vaporous carryover, which deposits on the turbine and causes erosion damage. Silica is the most notorious contaminant, and it is necessary to measure it in the boiler water and steam. Salts such as sodium hydroxide and ammonia salts also vaporize in the steam and flow into the turbine where they precipitate, concentrate, and become highly corrosive. To control contamination in the steam, the conductivity measurement of the boiler water must happen, which indirectly measures the dissolved solids. Then, blowdown controls the amount of contamination.

So, to avoid the uncontrolled corrosion that costs the power industry billions of dollars every year, monitor water quality rigorously and control that quality continuously.

Liquid analysis systems and sensors are hard working, easy-to-use, cost effective tools when measured against the impact of corrosion on plant costs and operations.

While every plant is different, generally an array of pH and conductivity sensing instruments is required for virtually every step of the steam-power generation process.

Beyond that, individual plants will require dissolved oxygen, ozone, chlorine, and other more specialized measurements.

Many plants are opting for centralized digital control systems to continuously monitor the output of analyzers and automate many control functions. This reduces impact on staff and allows corrosion control management to run like a well-oiled machine.

Most important, the key to successful corrosion control is the continuity of measurement.

Grab samples and other periodic measurement techniques are inadequate to the task. Only continuous, real-time analysis offers the assurance of water quality that corrosion control requires.

Sensing pH a venerated pursuit

In the sixteenth century, alchemist Leonard Thurneysser discovered that the hue of violet sap changed with the addition of either sulfurous or sulfuric acids. This early indicator was widely used through the subsequent centuries to detect acids.

With Svante Arrhenius's introduction of ionic theory in the 1880s, the first theories concerning disassociation of acids and bases were developed. Johannes Bronsted, who postulated that acids and bases are substances capable of either donating or accepting hydrogen ions, further refined these initial theories.

By 1904, Hans Friedenthal had successfully established the first scale for classifying acids by determining the dissociation constants for weak acids, according to conductivity and correlating color changes corresponding to different hydrogen ion concentrations using 14 indicating dyes.

The hydrogen ion concentration numbers from Friedenthal's calculations were small and awkward to manipulate. Thus, Lauritz Sorensen suggested using the negative logarithm of these numbers, which he dubbed the "hydrogen exponent" or "pondus Hydrogennii."

This led to the development of the term pH and the creation of the modern pH scale.

Modern pH Scale
The modern pH scale

 

 

Originated published at: https://www.isa.org/standards-and-publications/isa-publications/intech-magazine/2005/may/sensing-ph-controlling-ph/